Effect of dietary fibre on the energy value of feeds for pigs

Dietary fibre (DF) is an inevitable component of organic matter in pig feeds since it is present in most ingredients and to a high extent in by-products (wheat bran or corn gluten feed, for instance) or forages which are more commonly used. In growing pigs, digestibility coefficients of DF average 0.40–0.50 but they range from around zero in high lignin and water-insoluble DF sources (e.g. wheat straw) to 0.80–0.90 in fibre sources with high pectin or water-soluble DF levels (e.g. sugar beet pulp or soybean hulls). This also means that components of DF are digested differently by pigs: lignin is undigested while pectins are almost totally digested; hemicellulose tends to be more digested than cellulose, although both are partly digested. However, even if it is partly digested, DF provides negligible amounts of digestible or metabolisable energy to the growing pig due to increased endogenous protein and fat losses and negative interactions between DF and other dietary components. Digestive utilisation of DF improves with body weight of the pig with the highest values obtained in adult sows. However, this improvement is dependent on the botanical origin of DF. Consequently, DF makes a positive contribution to energy supply in adult sows and it is therefore recommended to give at least two energy values for pig feeds: one for growing pigs and one for adult sows. Digestion of DF is also associated with energy losses as methane. The efficiency of utilisation of metabolisable energy for net energy is poorer when it originates from DF (0.50–0.60 versus 0.80 for starch). Finally, the actual contribution of dietary fibre to energy balance of the pig can be affected by climatic conditions (heat increment of DF is used for thermoregulation) or changes in behaviour of pigs (lower physical activity with higher DF supply).